This invention relates to a method of printing characters on the surface of resin members, i.e., key tops used for various keyboards and cases.
On the key tops used for a keyboard, characters, symbols, drawings, etc. representing the functions of the key are often written. The characters, symbols, drawings, etc. (hereinafter generically referred to as "characters") on key tops are very frequently touched by the hand, so that they are required to have high durability. That is, the character on each key top should not be erased regardless of the frequency of touching the key top.
Usually, the following three different methods
have been used heretofore to form characters on key tops.
(1) Two-color molding method: As shown in FIGS. 1A and 1B, a key top 3 consisting of a base member 1 and a cover member 2 is molded by a double molding process using resin materials having different colors for the members 1 and 2 such that the base member 1 has an integral top projection 1a having a plan shape of a character in flush with the cover member 2 defining the background of the character. The top projection 1a of the base member 1, which consists of ABS resin, for instance, has the shape of a predetermined character on top of the key top 3.
(2) Sublimation ink printing method: As shown in FIGS. 2A and 2B, a key top 3 is molded using PBT resin, for instance. Then, the top of the key top 3 is printed with a predetermined character using a sublimation ink. The printing is effected by thermal permeation of the ink (to a depth of approximately 40 .mu.m) into the resin. After the printing, the key top is subjected to a thermal treatment (at approximately 170.degree. C. for 6 minutes) and a cleansing step, thereby obtaining a character pattern 4.
(3) Clear hard coat method: As shown in FIGS. 3A and 3B, a key top 3 is molded using ABS resin, for instance. Then a character is printed on the top of the key top 3 by about twofold overlap printing using a quick-drying ink. After the printing, the entire outer surface of the key top 3 is covered with a layer 5 of a multifunctional acrylic ultraviolet-ray-curable paint. The layer 5 is then subjected to polymerization hardening by irradiation of ultraviolet rays.
With any of the above prior art methods, satisfactory results can be obtained so far as the life of the character pattern is concerned, with the initial shape of the character being maintained after ten million times of striking of the key.
However, short production time, large-variety small-lot and low price tendency can not all be satisfied simultaneously.
(1) Drawbacks of the two-color molding method: With this method, the material cost is low because the inexpensive ABS resin is used. Also, the price of the product is comparatively low because the key top can be formed by the simple double molding.
However, since the base member 1 and cover member 2 are prepared separately, many molds are necessary, and the cost for the molds is increased. Further, the preparation of molds requires a long time, thus dictating a long period from the key top purchase order acceptance until the delivery of the product. Further, in the case of large-variety small-lot products, the cost is increased because of the cost of the molds. Furthermore, since the shape of the character itself is molded using a resin, the preparation of molds for complicated characters such as Chinese characters is difficult.
(2) Drawbacks of the sublimation ink printing method: The price of the product is comparatively high because a long thermal treatment time is necessary, and also because a step of wiping out residual ink on the key top surface is necessary. In addition, the color of the character has to be considerably thicker or darker than the color of the resin material constituting the key top 3. For example, a white character can not be printed on a black surface. Further, the PBT resin and ink material are comparatively expensive.
(3) Drawbacks of the clear hard coat method: The printing with the quick-drying ink has to be carried out through two-fold or three-fold printing, so that it is time-consuming. In addition, inperfect printing is liable to occur, and the yield is inferior. Further, an expensive clear hard coating (i.e., multifunctional acrylic ultraviolet-ray-curable paint) is coated over the entire outer surface of the key top 3. The coating operation is rather time-consuming. Further, because the clear hard coating is used, dust particles on the key top surface and in the coating are conspicuous, thus reducing the yield. Therefore, this method leads to the highest product price among the three methods noted above.